The present invention relates to inserting a geostationary satellite component into a non-geostationary satellite network. In particular, the invention relates to increasing the available user information bandwidth in a non-geostationary satellite network by redirecting administration information (for example, status and control information) through a geostationary satellite component.
Satellites are a common feature in modern communications networks and have long provided communications services on a global scale. A communications satellite often flies in a geostationary orbit (at approximately 42,245 km with an inclination and eccentricity of zero) so that the satellite always appears in the same spot in the sky. Satellites, however, may also be placed in other orbits, including Low Earth Orbits (LEO).
A LEO satellite typically orbits between 250 and 1000 km above the Earth. LEO satellites orbit the Earth independently of the Earth's own rotation and therefore do not maintain a constant location in the sky. Because the orbit of a LEO satellite periodically takes the LEO satellite over various locations on the Earth, the LEO satellite may be used to provide periodic communications services to those locations. A constellation of many LEO satellites may be used to provide nearly continuous coverage to virtually all areas of the Earth.
As an example, Teledesic LLC, located in Kirkland Wash., United States, has proposed a LEO constellation referred to as the Teledesic Network which flies 288 LEO satellites. The Teledesic Network incorporates 12 longitudinal orbital planes each with 24 LEO satellites. Each orbital plane is substantially perpendicular to the equator and separated from adjacent orbital planes by approximately 30 degrees. The altitudes of the satellites in each orbital plane are staggered so that the satellites pass below and above one another at the North and South poles, where each orbital plane converges. Although the discussion below is directed toward the Teledesic Network (and LEO satellite networks in general), it is noted that the present invention is applicable to any satellite network through which administration information, including status and control information, passes.
Two sets of optical links connect the satellites in the Teledesic Network. Sets of North-South links provide continuous connections between the satellites in individual orbital planes. Any first satellite in an orbital plane has connection to a second satellite ahead of its current position and a third satellite behind its current position. The North-South links provide a stable connection because the satellites in a particular orbit plane maintain substantially the same distance and angle between each other throughout their orbits.
Similarly, a set of East-West links provides a connection between the satellites in a first orbital plane and the satellites in a second orbital plane and a third orbital plane on either side of the first orbital plane (the adjacent orbital planes). Near the North and South Poles, however, the satellites typically do not maintain their East-West links due to dramatic increases in the relative rates of motion between adjacent satellites (slew) and because of the adverse pointing angles required to align receivers and transmitters. The satellites reestablish their East-West links after passing over the poles. Therefore, during the time periods in which the satellites pass over the Poles, little, if any, East-West communication occurs.
The North-South and East-West links (collectively "links") create a connective mesh that moves with the satellite network. The connective mesh routes data between individual satellites so that information injected into the satellite network from the ground may make its way to a satellite flying over the destination geographic region.
The links thus allow information to flow from virtually any point on the ground to any other point on the ground. However, because the satellites are constantly moving with respect the Earth, the connectivity dictating an optimal path from point to point on the surface of the Earth is constantly changing. As a result, a single, or small number of, ground cased Network Control Centers (NCCs) frequently transmit updated routing tables to the satellites. The satellites thus frequently update their routing tables so that they may provide an efficient information transport mechanism from source to destination as their positions continually change. Furthermore, the NCC is responsible for forwarding administration information through the satellite network to ground stations.
The administration information includes status and control information, for example, routing tables, cellular phone call-setup, and call-teardown. In general, administration information passes between ground stations and satellites on radio frequency (RF) links. The RF links may use a portion of the Ka frequency spectrum, for example, a 23-29 GHz uplink and a 18-19 GHz downlink. The RF links, of course, are also used to communicate user information from the ground to the satellite network and from the satellite network down to the ground.
Ordinarily, an NCC injects and receives status and control information into the satellite network by transmitting the status and control information to a satellite passing over the NCC. The status and control information is then forwarded through the network using the links and, when necessary, transmitted down to a destination ground station by a satellite flying over the destination ground station. A destination ground station may, for example, be a simple cellular phone or a complex communications center.
The bandwidth used to communicate the status and control information to and from the ground may grow very large as additional users take advantage of the satellite network. The status and control information may therefore significantly reduce the bandwidth available for user information and therefore the total capacity and revenue generating potential of the satellite network. In other words, the regular user information flow to ground based locations and around the satellite network is generally disrupted by the status and control information. In order to avoid disrupting traffic in major revenue producing regions, the NCC must be carefully located away from the revenue producing regions, potentially making NCC access relatively inconvenient for maintenance and upgrades, for example.
Furthermore, when a satellite in the network fails, it can no longer function as part of the connective mesh or handle status and control information. Therefore, some provision must be made to forward the status and control information into the network through an alternate NCC until such time as the initial NCC can again connect to the network. A failed satellite creates a hole in the connective mesh requiring further updates to the routing tables transmitted to the satellites.
A need exists in the industry for an improved method of handling control and status information for satellite networks.